Temperate grazed grassland carbon balances for two adjacent paddocks determined separately from one eddy covariance system

Abstract

Use of eddy covariance (EC) as part of net ecosystem carbon balance (NECB) studies of grazed grasslands has the potential to provide valuable insights into the influence of management on soil carbon (C) stocks and opportunities for potential mitigation. However, in rotationally grazed systems where the EC measurements extend across multiple paddocks, often due to necessity, interpretation of results can be complicated by differing paddock-scale management regimes despite being the same land use with a single overarching farm-scale management approach. Here, we calculated separate annual CO2 fluxes and NECBs for two adjacent paddocks under the same general land management using a single EC system for seven years. We were interested in determining whether fluxes and annual NECB of the paddocks were sufficiently similar to allow future side-by-side comparisons of management practices aimed at increasing soil C. We assigned the half-hourly flux measurements to individual paddocks (P31 and P32) using footprint analysis and then gap filled each dataset separately. Additionally, we calculated annual fluxes (and NECBs) for the integrated area of both paddocks (i.e. using all measured flux data; referred to as NewMix). The mean (± 95% confidence interval) NECB for P31 and P32 were 1 ± 76 g C m−2 y−1, and 7 ± 80 g C m−2 y−1, which compared well with the NECB of NewMix (–14 ± 48 g C m−2 y−1). Large between-paddock and inter-annual variability were observed, and several consecutive years of data were required to determine with high certainty that both paddocks were C neutral. Day-to-day management decisions were identified as causing some of the variability. We concluded that the calculation of fluxes and NECB's from two adjacent paddocks using a single EC system was possible, and indeed, likely more insightful and useful than integrating across multiple paddocks. Such an approach can provide opportunities to more accurately quantify management effects, and test mitigation strategies by using treatment and control experiments on adjacent paddocks using a single EC system, although multiple measurement years are likely required. Alternatively, using two EC systems at separate sites, each quantifying both treatment and control fluxes would allow for replication, and thus greater statistical power, which has been a limitation for many EC based NECB studies.